Abstract West Nile virus (WNV), a category B NIAID priority agent, is a neurotropic flavivirus that is the leading cause of mosquito-borne encephalitis of humans in the United States. The continuing spread of WNV, combined with the lack of specific therapeutics or vaccines to combat or prevent infection, imparts a pressing need to identify the viral and host processes that control infection and immunity. The pathogenesis of WNV in humans is poorly defined, but a mouse model of WNV infection, which faithfully recapitulates the major phases of WNV pathogenesis observed in humans, has provided significant insights into the mechanisms that cause WNV disease. The innate immune response (mediated by RIG-I like receptor [RLR] signaling) and the humoral and cell-mediated responses are critical for protection against WNV infection. The RLR signaling pathway functions to trigger antiviral immune defenses and program protective immunity during WNV infection. Our studies have recently revealed a novel connection between RLR signaling and regulation of CD8+ T cell immunity during virus infection. In support of our findings, other groups have now implicated MAVS, the central adaptor protein required for RLR-mediated innate immune signaling, with regulation of CD8+ T cell responses during chronic viral infection, driving CD4+ T cell polarization during bacterial infection, and regulating TH1 and TH17 responses during autoimmune encephalitis, thus demonstrating the importance of RLR signaling in programming T cell immunity. However, the immunological mechanism underlying MAVS regulation of T cell immunity are not well understood. We seek to fill this gap in our knowledge by developing a new research tool to study MAVS immune regulation in a cell and tissue-specific manner. In Aim 1, we will generate Mavsfl/fl mice that can be crossed with cre-expressing mice to ablate MAVS expression in a context-dependent manner. In Aim 2, we will generate MAVS-DC KO and MAVS-CD8 KO mice, which lack MAVS expression in dendritic cells (DCs) and CD8+ T cells, respectively. We will use these mice to evaluate protection against WNV infection, control of viral pathogenesis, and development of CD8+ T cell immune responses. The results from this study will reveal the immunological mechanisms underlying MAVS-mediated immune regulation of T cell immunity during WNV infection. These studies will potentially uncover the therapeutic and immune-modulating potential of the RLR signaling pathway during virus infection and vaccination. !!